1. Field of the Invention
The present invention relates to a water electrolysis system and a method for shutting down the system.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell generates direct-current electric energy by supplying a fuel gas (gas mainly containing hydrogen, for example, hydrogen gas) to the anode-side electrode and an oxidizer gas (gas mainly containing oxygen, for example, air) to the cathode-side electrode.
In order to produce hydrogen gas as the fuel gas, a water electrolysis apparatus is generally used. The water electrolysis apparatus uses a solid polymer electrolyte membrane (ion-exchange membrane) for generating hydrogen (and oxygen) by water decomposition. In addition, electrode catalyst layers are provided on both surfaces of the solid polymer electrolyte membrane to form an electrolyte membrane/electrode assembly. Further, power feeders are disposed on both sides of the electrolyte membrane/electrode assembly to form a unit. That is, the unit has substantially the same configuration as the fuel cell.
Therefore, in a stack of a plurality of units, a voltage is applied across both ends in the stacking direction, and water is supplied to the anode-side power feeder. As a result, hydrogen ions (protons) are generated by water decomposition on the anode side of the electrolyte membrane/electrode assembly, and the hydrogen ions permeate through the solid polymer electrolyte membrane, move to the cathode side, and combine with electrons to produce hydrogen. On the other hand, on the anode side, oxygen produced together with hydrogen ions (protons) is discharged from the unit accompanied by excess water.
A hydrogen supply system disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2006-131942 is known as such a type of water electrolysis system. The hydrogen supply system is provided with at least one hydrogen/oxygen generator configured to have an electrolysis cell in which the anode side and the cathode side are separated by a diaphragm so that hydrogen gas is generated on the cathode side and oxygen gas is generated on the anode side by electrolysis of water supplied to the electrolysis cell.
In addition, the hydrogen supply system is configured so that at least the hydrogen gas of the hydrogen gas and oxygen gas generated by the hydrogen/oxygen generator can be supplied to a point of use, and the pressure of the hydrogen gas generated at lower pressure than that of the oxygen gas in the system can be increased by the oxygen gas generated on the anode side of the electrolysis cell of the hydrogen/oxygen generator.
However, the above-described hydrogen supply system may employ a differential pressure-type hydrogen generation system in which the pressure on the cathode side where hydrogen gas is generated is set to be higher than the pressure of the anode side where oxygen gas is generated. This is because rapid hydrogen supply can be easily performed by handling as high-pressure hydrogen gas.
In this differential pressure-type hydrogen generation system, when electrolysis is stopped, high-pressure hydrogen gas is present on the cathode side, while normal-pressure water and oxygen gas are present on the anode side. Therefore, hydrogen easily permeates through the diaphragm and moves from the cathode side to the anode side during the time when the pressure on the cathode side is slowly released for preventing damage to a seal and MEA (Membrane Electrode Assembly) after electrolysis is stopped (so-called cross leak).